Squeegee type single-drum rotary printing devices are widely used as one type of single-drum rotary stencil printing devices, and comprise a cylindrical printing drum including an ink permeable outer shell on which a stencil master plate is mounted, and a squeegee member consisting of a squeegee roller or a squeegee blade engaging with the inner surface of the outer shell of the printing drum. As the printing drum or, in particular, the outer shell rotates, printing ink is squeezed through or across the ink permeable outer shell by the squeegee member which is stationary.
According to the conventional printing drum structure disclosed in Japanese patent laid-open publication (kokai) No. 3-254985, the two axial ends of the outer shell are closed by end walls, and the outer shell is rotatably supported by a central shaft passed through the central parts of these end walls. The squeegee member is supported by the central shaft.
According to the alternate conventional structure disclosed in Japanese patent laid-open publication (kokai) No. 59-12893, the printing drum having an open end is rotatably supported at its outer circumference adjacent to its open end by a fixed side plate in the manner of a cantilever, and a fixed frame securely attached to the side plate projects into the interior of the printing drum. The squeegee member is mounted on this fixed frame for engagement with the inner surface of the outer shell of the printing drum.
Single-drum rotary stencil printing devices not using any squeegee member are also known, and printing drums having a central shaft for use in such stencil printing drums are disclosed in Japanese patent laid-open publications (kokai) Nos. 56-72987, 62-42873, 59-12893 and 2-164576. According to this group of conventional stencil printing devices, the two ends of the printing drum are closed by end walls, and the central shaft is passed centrally through these end walls.
According to the printing drum having end walls and a central shaft passed through these end walls, because the printing drum is supported at its two axial ends, the printing drum is relatively free from any eccentricity or mis-alignment in the overall structure, and can be supported in a stable fashion. Furthermore, because the interior of the printing drum is entirely enclosed by the end walls, the printing ink is favorably confined within the printing drum, and would not leak out even during transportation of the printing drum. However, the enclosed structure of the printing drum would not allow easy access to the interior of the printing drum, and an ink bottle serving as a source for printing ink cannot be readily replaced. Also, when the squeegee angle and the squeegee pressure are to be adjusted, the printing drum has to be dismantled. Normally, it is not possible to adjust the squeegee angle and the squeegee pressure while the printing drum is mounted on the stencil printing device.
According to the cantilever type printing drum for a stencil printing device, because one of the axial ends of the printing drum is open, an ink bottle placed inside the printing drum can be readily replaced, and the squeegee angle and the squeegee pressure can be adjusted while the printing drum is mounted on the stencil printing drum. On the other hand, due to the nature of the mode of supporting the printing drum, it is not easy to support the printing drum in a stable fashion, and there is an increased possibility of involving eccentricity. If there is an excessive eccentricity in the printing drum, it will cause unevenness in the density of the printed images. Furthermore, there is an increased possibility of causing leakage of printing ink from the interior of the printing ink, in particular when the printing drum is placed vertically with its open end down. Leakage of printing drum should be avoided because it will smear the clothing of the operator and other parts. Furthermore, because the interior of the printing drum consists of a single chamber, and there is no separating wall between the squeegee member assembly and the ink bottle/pump assembly, printing ink which has leaked from the squeegee member assembly may smear the various parts of the ink bottle/pump assembly. Possibility of smearing the clothing of the operator will seriously reduce the acceptability of the stencil printing device for office use.
According to the printing drum for a single-drum stencil printing device not using a squeegee member, the printing drum is substantially enclosed with the two axial ends of the printing drum closed by end walls, and a central shaft is passed through these end walls. However, the printing ink used for such printers has a relatively low viscosity, and tends to seep through the outer shell of the printing drum under the action of gravity. Therefore, according to this type of printing drums, the rest position of the printing drum must be selected such that a non-ink permeable part of the printing drum be located at the bottom end of the printing drum whenever the printing drum is brought to a stop.